Over the years, architectural materials, such as metal roofing systems and metal siding systems, made of pliable metals in various sheet gauge thicknesses have been used. Metals such as carbon steel, stainless steel, copper and aluminum are the most popular types of metal. These architectural metal materials are commonly treated with corrosion-resistant coatings to prevent rapid oxidation of the metal surface, thereby extending the life of the materials. A popular corrosion-resistant coating for carbon steel and stainless steel is a terne coating. Terne coating has been the predominate and the most popular coating for roofing materials due to its relatively low cost, ease of application, excellent corrosion-resistant properties and desirable colorization during weathering. The terne coating is an alloy typically containing about 80% lead and the remainder tin. The coating is generally applied to the architectural materials by a hot-dip process wherein the material is immersed into a molten bath of terne metal. Although terne coated sheet metals have exhibited excellent resistant properties and have been used in a variety of applications, the terne coating has been questioned in relation to its impact on the environment. Environmental and public safety laws have been recently proposed and/or passed prohibiting the use of materials containing lead. Because the terne alloy contains a very high percentage of lead, materials coated with terne have been prohibited in various types of usages or applications such as aquifer roofing systems. The concern of lead possibly leaching from the terne coating has made such coated materials inadequate and/or undesirable for several types of building applications. The terne alloy has a further disadvantage in that the newly applied terne is very shiny and highly reflective. As a result, the highly-reflective coating cannot be used on buildings or roofing systems such as at airports and military establishments. The terne coating eventually loses its highly-reflective properties as the components within the terne coating are reduced (weathered); however, the desired amount of reduction takes approximately 1 1/2 to 2 years when the terne coating is exposed to the atmosphere, thus requiring the terne metals to be stored over long periods of time prior to being used in these special areas. The storage time is significantly prolonged if the terne-coated materials are stored in rolls and the rolls are protected from the atmosphere.
Tin coating of carbon steel is a well-known process for use in the food industry. However, in the specialized art of architectural materials, a tin coating for architectural materials has not been used until just recently as disclosed in U.S. Pat. No. 5,314,758. The most popular process for applying a tin coating to carbon steel for use in the food industry is by an electrolysis process. In an electrolysis process, the coating thickness is very thin and typically ranges between 3.8.times.10.sup.-4 to 20.7.times.10.sup.-4 mm (1.5.times.10.sup.-5 to 8.15.times.10.sup.-5 in.). Furthermore, the equipment and materials needed to properly electroplate the metal materials are very expensive and relatively complex to use. The expense of applying an electroplated-tin coating and the limited obtainable thicknesses of the tin coating are a disadvantage for using such a process for building and roofing materials.
A hot-dip process for applying the tin coating may be used; however, if the architectural materials are not properly prepared and the coating is not properly applied to the roofing materials, minute areas of discontinuity in the tin coating may occur resulting in non-uniform corrosion protection. This is especially a problem when the tin is applied to stainless steel materials by a hot-dip process. Tin is not electroprotective to steel under oxidizing conditions. Consequently, discontinuities in the tin coating result in the corrosion of the exposed metal. Tin coatings have the further disadvantage of having a highly-reflective surface. As a result, architectural materials coated with a tin coating cannot be used in an environment where highly-reflective materials are undesirable until the coated materials are further treated (i.e. painted) or the tin is allowed time to oxidize.
Coating architectural materials with zinc metal, commonly known as galvanization, is another popular metal treatment to inhibit corrosion. Zinc is a highly desirable metal to coat architectural materials with because of its relatively low cost, ease of application (i.e. hot-dip application) and excellent corrosion resistance. Zinc is also electroprotective to steel under oxidizing conditions and prevents the exposed metal, due to discontinuities in the zinc coating, from corroding. This electrolytic protection extends away from the zinc coating over exposed metal surfaces for a sufficient distance to protect the exposed metal at cut edges, scratches, and other coating discontinuities. With all of the advantages of using zinc, zinc coatings have several disadvantages that make it undesirable for many types of building applications. Although zinc coatings will bond to many types of metals, the formed bond is not strong and can result in the zinc coating flaking off the building materials. Zinc does not bond well on standard stainless steel materials. Zinc does not form a uniform and/or thick coating in a hot-dip process for stainless steel. As a result, discontinuities of the coating are usually found on the stainless steel surface. Zinc is also a very rigid and brittle metal and tends to crack and/or flake off when the building materials are formed on site, i.e. press fitting of roofing materials. When zinc begins to oxidize, the zinc coating forms a white powdery texture (zinc oxide). The popular grey, earth tone color is never obtained from pure zinc coatings.
Electroplating a tin and zinc mixture onto a steel sheet is disclosed in Japanese Patent Application No. 56-144738 filed Sep. 16, 1981. The Japanese patent application discloses the plating of a steel sheet with a tin and zinc mixture to form a coating of less than 20 microns thick. The Japanese patent application discloses that after plating pin hole exist in the coating and subject the coating to corrosion. The pin holes are a result of the crystalline layer of a tin and zinc mixture which slowly forms during the plating process. The charged tin and zinc atoms in combination with the atomic structure of the atoms and formed crystal structure of a tin and zinc mixture prevents a uniform coating from being formed on the plated steel sheet. Consequently, the crystalline depositions must be covered with a chromate or phosphoric acid to fill the pin holes and prevent immediate corrosion. The Japanese patent application also discloses that a preplated layer of nickel, tin or cobalt on the steel sheet surface is needed so that the plated tin and zinc mixture will adhere to the steel sheet. Such electroplating techniques as disclosed in the Japanese patent application cost a tremendous amount of time and money and are not a commercially successful product.
The coating of steel articles with a tin, zinc and aluminum mixture is disclosed in U.S. Pat. No. 3,962,501 issued Jun. 8, 1976. The '501 patent discloses that the tin, zinc and aluminum mixture resists oxidation and maintains a metallic luster. The '501 patent discloses that the coating is applied by immersing a steel article into the molten alloy bath and subsequently withdrawing the steel article. The '501 patent also discloses that a molten tin-zinc alloy bath containing 3-97% zinc is very susceptible to oxidation at the surface thus producing viscous oxides which causes severe problems with the process of immersing the steel articles into the molten alloy and subsequently removing the steel article from the molten alloy. Further, while the steel article is in the molten alloy, a large amount of dross is produced which results in non-uniformity of the coating and formation of pin holes. The '501 patent discloses that the addition of up to 25% aluminum to the tin and zinc mixture inhibits dross formation during immersion of the steel article, prevents Zn-Fe alloy formation and reduces the viscous oxide formation on the molten bath surface. The '501 patent does not teach the use of a continuous, hot dip coating process which resolves the viscous oxide problem and dross formation problem. The '501 patent also discloses the formation of a highly reflective coating which cannot be used in many building applications.
Due to the various environmental concerns and problems associated with corrosion-resistant coatings applied to metal architectural materials, there has been a demand for a coating which can be easily and successfully applied to materials that protect the materials from corrosion, does not have a highly-reflective surface subsequent to application, can be applied by a continuous hot-dip process, weathers to a grey, earth tone color and allows the materials to be formed at the building site.